Sleep and circadian rhythms are increasingly recognized as important determinants of health and functioning. Poor sleep health and misaligned circadian rhythms increase the risk of adverse health outcomes such as depression, cognitive impairment, cardiovascular disease, diabetes, and even mortality. However, we know very little about how sleep and circadian disruption lead to health risks later in life. We propose to study retired night shift workers (RNSW) and retired day workers (RDW) >60 years of age as a model to understand the effects of repeated sleep and circadian disruption on sleep, circadian, and cardiometabolic health in later life. Approximately 15% of the US population are retired shiftworkers. In previous studies we have shown that, compared to RDW, RNSW have worse subjective sleep quality; worse polysomnographic sleep; evidence of circadian rhythm abnormalities; and increased rates of diabetes and obesity. Additional new preliminary data with an independent sample suggest that, compared to day workers, older shift workers also have worse outcomes on cardiometabolic risk factors including endothelial dysfunction, poor glucose control, and increased prevalence of the metabolic syndrome. The proposed study will advance these preliminary findings in several important ways: First, we will measure not only PSG sleep, but also the homeostatic regulation of sleep following sleep deprivation; second, we will characterize circadian phase, amplitude, and phase angles using a constant routine laboratory protocol; third, we will use a multi-dimensional approach to assess intermediate markers of cardiometabolic health including metabolic syndrome, brachial artery flow- mediated dilation, and carotid intima-media thickness. We will study 45 RNSW and 45 RDW in an intensive sleep/circadian laboratory protocol, and 80 RNSW and 80 RDW in an assessment of cardiometabolic health. We hypothesize that, compared to RDW, RNSW will show: 1) Reduced sleep efficiency and smaller slope of delta power across NREM sleep; 2) More aberrant values in the phase angle between dim-light melatonin onset and mid-sleep time, and lower melatonin amplitude; and 3) Increased prevalence of metabolic syndrome, decreased flow-mediated dilation, and increased intima-media thickness. Exploratory analyses will evaluate the extent to which these effects are moderated by individual-level variables (race, age, sex, socioeconomic status, shift work exposure variables), and dynamic associations among endothelial function, homeostatic sleep drive, and circadian rhythms. This study is significant because of the prevalence and known health consequences of shift work, coupled with an aging population; plausible based on published literature and preliminary findings; feasible based on previous studies; and innovative because it uses a multimodal assessment of physiological function and regulation in sleep, circadian, and cardiometabolic domains. This study will impact our understanding of the health consequences of shift work, which may point to future intervention and rehabilitation strategies.